[EMAIL PROTECTED] wrote: >>With the experience I have gained so far, it leads me to believe that >>you are making choices that may make your quest more difficult. If you >>are trying to have a CNC machine to make parts and spend as little as >>appropriate for that end, then you should consider studying other >>machines that are already proven to perform and copy their success. >> >>If you want to explore different methods of machine control, be prepared >>to spend allot more time, money and run into dead-ends. You will also >>need to be able to develop your own software because these days, >>hardware won't do anything without software. >> >> >whoa there, are you telling me EMC can't do PID? > >shirley not.......... > > EMC can do PID just fine. It's steppers that can't. Steppers lose torque as the speed increases. There is no way around this, it's just the physics of the motor. A PID loop will attempt to correct for a lagging motor by requesting more "effort" from the motor. A servo can do this - more current = more torque, so a bigger command output says "push harder, dammit!". With a stepper, the current is fixed at whatever the max setting is on the driver. Asking the motor to go faster reduces torque (push), so a lagging motor is already lost - once it's missed a step you probably need to stop the machine and ramp up to velocity again. Even if the motor just loses a step or two which is detected by the scale, you can't get it to catch up - it's already at the limit of its power envelope or it wouldn't have fallen behind in the first place.
You had an incorrect assumption in your original email: that using linear scales will eliminate backlash issues. This isn't true at all. Backlash is an uncertainty in machine position. If you're climb milling, the cutter will tend to pull the table "ahead" of the motor. When conventional milling, the cutter will resist motor motion. It's not possible for the control to know which type of cutting is taking place at any given time, and it may even vary within a move, so there's no way to "compensate" for it. Additionally, de-coupling the feedback from the motor, especially through a drive with backlash, will make the system very hard to tune. The PID integrator will "wind up" as the motor starts to spin to take up the backlash, but the feedback won't change until the motor is already moving. The motor will slam the table into motion, at which time the PID starts to wind up the other way. The result is - you guessed it - oscillation. This is very hard to tune out. There has been some discussion recently about using both encoders and linear scales, but there isn't any software to do that yet. I think this is the "different method of machine control" that Kirk is talking about. As for redundancy, since EMC takes encoder feedback, there isn't really any need for a DRO - the EMC display is actual position. - Steve ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
